Traditional aerated concrete as well as similar types of porous steam cured light weight concrete, e.g. foam concrete, is usually produced in the following way. One or several silica containing materials, such as sand, shale ashes or similar materials, as well as one or several calcareous, possibly hydraulic, binders, such as lime and/or cement, are mixed with a rising agent--which in the case of aerated concrete is aluminium powder--in water. When these materials are homogenized and in close contact, the lime (CaO) of the binder reacts with the water and the silica (SiO.sub.2) material and forms what can generally be described as a calcium silicate hydrate mass at the same time as the aluminium powder reacts with water to develop hydrogen gas that gives the mass macroporosity. This pore formation means in practise that the mass rises from a limited initial volume to a relatively large volume. (The finished aerated concrete usually has a density of 0.4-0.65 kg/dm.sup.3).
In practise, the rising of the mass forming components takes place in a special mould, into which they are poured from a special mixing device. After rising, the mass is allowed to stiffen in the mould during a special stiffening time. During this stiffening time a semiplastic body is formed which has a relatively low strength but which is sufficiently stiff to keep together without support from the mould and can be transported on its own. As soon as this stiffness is achieved, the body is released from the mould, whereupon the body, in one way or the other, is divided by cutting devices such as wires into separate elements having shapes that are suitable for use in the building industry. The divided body is brought to an autoclaving station in which it, for a certain period of time, is steam cured at high pressure and high temperature (170.degree.-200.degree. C.) in order to obtain suitable strength. Finally the body is transported from the autoclaving station to an unloading station in which the elements of each body are separated from each other and packaged and/or transported to a dispatch place.
During the above mentioned reaction between the calcareous binder and water, heat is developed. Professionals have always tried to keep this heat at the lowest possible level since a too fast a temperature increase in the mass produces difficulties in controlling the rising process. For this reason, hard burnt lime, which reacts slowly with limited development of heat, has been used throughout for the manufacture of aerated concrete on the basis of lime. This has been the case, independently of whether lime has been included in the formula as the sole binder or has been mixed with larger or smaller amounts of portland cement.
Although the use of hard burnt, slow reacting lime has ensured good control during the casting/rising process, the investigations which lead to this invention have shown that a multitude of inconveniences are connected with the use of slow reacting binders, especially if the manufacture of aerated concrete is seen from a wider perspective. A not insignificant inconvenience is that hard burnt lime is considerably more expensive than soft burnt, highly reactive lime of the type that, e.g., is used in steel manufacture (hereunder called "Steel works lime"). Another inconvenience--with more profound consequences--is that the slow reacting lime gives a relatively long stiffening time. Thus the stiffening time is usually more than 45 minutes whereas the pacing time, i.e. the time between two consecutive castings, often is as short as four to five minutes in order to obtain a high production capacity. This means, of course, that the production or manufacturing plant must have at least 12-15 moulds that are working at the same time since the body cannot be released from the mould and be divided until the necessary stiffening time is ended. In practice, however, the number of moulds is considerably greater, e.g. 20 or more, in order to fill the demands on spare- or buffer capacity.
Moulds are expensive to manufacture as well as to maintain and a considerable amount of space in the factory is necessary, something that leads to high investment costs and running costs. High investment costs mean that every plant must have high production capacity; all this has, in practice, led to erection of big, highly effective factory units, from which large amounts of different products are delivered. Manufacturing aerated concrete in this manner, however, becomes more and more untenable, especially in sparsely populated areas, where high transportation costs in combination with high investment costs make the cost for delivery of the elements, capital cost included, unacceptably high.